Visualization of IR radiation in the atmospheric IR window of 8-12 μm (and in principle in a wider spectral range) is achieved in the technology by projecting an IR picture onto a sufficiently large (up to hundreds of thousands of pixels) 2D matrix (array) of small square sensing microbolometers, typically from 25×25 μm2 to 50×50 μm2 each, this array being placed in the focal plane of IR optics projecting the picture. For a fixed period of time referred to as the frame (typically in the 1-30 ms range) an IR picture is projected onto an array, exposing it to spatially non-uniform intensity of IR radiation. Each pixel integrates the IR radiant energy it receives and, provided it is thermally isolated from a heat-sunk substrate, reacts to the energy influx by raising its temperature. In the prior art thermal isolation is achieved by placing individual sensing pixels onto silicon nitride membranes (micro bridges) suspended above a substrate, and by evacuating the package to eliminate heat conduction through air. Provided that sensor resistivity is temperature-dependent, a change in pixel's temperature in turn produces a change in its electrical resistance. The two electrical leads applied to each sensing element provide for reading out the change in said element's electrical resistance. For example, in one implementation, this change in resistance is producing a current change at a constant-voltage pulsed bias applied for 70 μs to each microbolometer, thus providing a means of transforming an IR picture into a collection of electrical signals from the pixels. These electrical signals in turn can be visually displayed, thus reproducing the pixilated image of the original IR picture in the visible domain.
One of the issues resolved by the Invention relates to the sensor material and its electrical resistance as a function of temperature. The material of choice in the modern commercially available (focal plane array) FPA technology is a thin film (typically 50 nm) of vanadium oxide VOx. While VOx contains mostly VO2, it is not a pure-phase vanadium dioxide. Initially in the uncooled infrared imaging technology there were proposals to operate the uncooled (even heated) bolometer as a transition-edge device using the strong semiconductor-to-metal phase transition (SMT), such as found in VO2 at 68 C in single crystals and between 50 C and 90 C in typical polycrystalline films. SMT-based device was proposed as a high-temperature substitute for a superconducting transition-edge bolometer operating at low temperatures. Resistivity ρ changes by a factor of ˜103-104 in SMT in VO2 films, providing high temperature coefficient of resistivity TCR=(1/ρ)dρ/dT and thus holding a promise of high bolometer responsivity.
Although this attractive idea continues to reappear in the prior art, the modern practical implementation of the uncooled focal plane array (UFPA) infrared imaging technology is based on resistive readout of individual VOx microbolometers operating at or around room temperature, away from the SMT in VO2. Usually the non-stoichiometric VOx films used in this technology do not posses SMT at all. The reason for abandoning the very high TCR found in the transition region is that, it is accompanied by other undesirable features, such as hysteresis for example. There is also latent heat released/absorbed in the transition, which is feared to interfere with bolometer operation. Also the fact that transition takes place at elevated temperatures requires heating of the bolometer above the room temperature. There are also reasons to suspect that VO2 in the hysteretic transition region will exhibit an excess flicker 1/f noise resulting from electrons transitioning by tunneling or activated hopping between conductive (M) microdomains separated by semiconducting (S) microdomains. Indeed, it is known that mixtures of conductive and insulating domains are prone to such noise, which has been found, for example, in polymers filled with metallic particles. This excess noise in the transition region of VO2, has not been properly measured, but assumed to be there based on the physical picture of fluctuating M and S microdomains co-existing in the hysteretic region.
In view of the above, initial attempts to use the phase-transition were abandoned. Mixed vanadium oxide VOx with x≈2 was found to posses an attractive combination of reasonably high TCR=(1/ρ)dρ/dT and low R□=ρ/d at 25 C in the semiconducting phase [here ρ is resistivity, d film thickness, R□ is the resistance of a square (pixel)], as well as moderately low 1/f noise. Thus, vanadium oxide was considered a suitable semiconductor sensor material despite a decision to abandon its phase transition capability. The mixed oxide VOx used at ˜25 C in commercial UFPA bolometers may not even exhibit a phase transition at higher temperatures.
VOx is manufactured to provide TCR˜(−2%). In the prior art parameter of VOx sensor material R□ varies in the wide range, from 10 kΩ to 200 kΩ at 25 C. However, R□=10-20 kΩ is the preferred range in FPA applications, with higher R□ causing problems in readout and in terms of noise. With this limitation on R□, the use of high crystalline quality pure phase VO2, which would have higher TCR, is problematic: VO2 single crystals and epitaxial films have ρ(25 C) in the range 0.1 Ωm to 1.0 Ωm. This implies R□=2-20 MΩ for a 50 nm film thickness typically used in FPA sensors. These R□ values are 100-200 times higher than required. In the work of the inventors it was found that the room-temperature values of R□ for 50 nm pure-phase VO2 films were from about 1.5 MΩ to about 4.2 MΩ, while TCR varied from −2.5% to −5%. Despite an attractive TCR, the high R□ values should make these films unsuitable for the resistive-readout IR imaging application at or around 25 C.
An important issue discussed in the application is why high R□ is detrimental?
First, one needs to match the pixel resistance to the electronic readout circuit which is amplifying the small resistance change associated with the IR signal. This matching is apparently becoming more difficult at high R□.
A second reason why high R□ is detrimental to the FPA performance is the increase in Johnson's noise. Johnson's noise has been sited as the major contributor to overall noise even at R□=20 kΩ. Johnson's noise is one ingredient in more practical consideration of signal to noise ratio (SNR) in the device of the invention. Let us consider R□-dependence of signal to noise ratio in the prevailing measuring scheme, in which all pixels (sensors) are biased by the same constant voltage V0, and the change in each pixel's resistance ΔR□ is producing a change in individual pixel's current ΔI, the latter representing the useful signal.
By Ohm's law I=V0/R□ and, at constant V0, |ΔI|=(V0/R□2) ΔR□. At the same time Johnson noise manifests itself as fluctuations in V0, with the rms average of these voltage fluctuations δV0 being proportionate to R□1/2, according to δV0=(4kTΔf R□)1/2, where k is Boltzmann's constant, T absolute temperature, Δf measurement bandwidth. Therefore current noise will be equal to δI=δV0/R□=(4kTΔf/R□)1/2. The signal-to-noise ratio for the current S/N=|ΔI|/δI=[V0ΔR□/(4kTΔf)1/2]/R□3/2, and further replacing ΔR□/R□ with ΔT(TCR), we findS/N=|ΔI|/δI=[V0ΔT(TCR)/(4kTΔf)1/2]/R□1/2  (1)
As could be expected, in this voltage-bias measuring scheme, the SNR for the current is proportional to the voltage pulse amplitude, to the pixel's temperature change ΔT and to the TCR. However, this analysis also shows that it is proportional to R□−1/2, indicating that higher R□ corresponds to significantly lower SNR. For example, a factor of 100 higher R□ corresponds to 10 times lower SNR. Note that if SNR were defined as the ratio of corresponding powers, formula (1) would have to be squared, and in our example SNR power would be 100 times smaller.
The third reason for rejecting the high resistance pixels is: increased current (Joule) heating during readout.
One can not effectively resolve the difficulty of exceedingly high pixel resistance by making the sensing layers thicker, and thus reducing R□. One can not make them 100 thicker for technological reasons; While making films which are 2-3 times thicker is technologically feasible, any increase in sensor thickness is undesirable as it increases the bolometer thermal mass and thus reduces responsitivity. In this sense, an increase in thickness is equivalent to a reduction in TCR.
Clearly, in a semiconductor, the requirements of high TCR and low ρ (or R□) are directly conflicting with each other, making high TCR pure phase VO2 films unusable in FPA application due to their high resistivity.
If it were not for the large resistance, pure phase VO2 would be preferred over VOx in the near room temperature operation. This is because of higher bolometer TCR of 2.5-5% vs. 2% and because a well-defined single phase sensor material should provide for an easier process control compared to a need to reproduce and make uniform layers of a mixed, ill-defined, ill-behaved VOx. Furthermore, a pure phase sensor material with fewer defects should have a lower 1/f noise.
The present invention is based on inventor's discovery of a new phenomenon which takes place in pure phase VO2 and offers the possibility of preserving the high TCR, while avoiding hysteresis and dramatically, by orders of magnitude, lowering R□. Moreover, the explanations of such new phenomenon indicate that its use circumvents many other difficulties associated with the phase transition, namely, the emission/absorption of latent heat and excess noise.